1. Field of the Invention
The present disclosure relates to a light emitting device and a method of fabricating the same.
2. Description of the Background
A light emitting diode refers to a semiconductor device that has a p-n semiconductor junction, which emits light through electron-hole recombination. Such light emitting diodes are used in a wide range of applications, such as display devices, backlights, etc. Furthermore, light emitting diodes have a lower power consumption and longer lifetime than existing electric bulbs and fluorescent lamps. Thus, light emitting diodes are being implemented as a substitute for existing electric bulbs and fluorescent lamps used in general illumination.
In recent years, AC light emitting diodes, which are directly connected to an AC power source to continuously emit light, have been produced. One example of AC light emitting diodes that are directly connected to a high voltage AC power source is disclosed in PCT Publication No. WO 2004/023568 (A1), entitled “Light emitting device having light emitting elements” by Sakai et al.
According to PCT Publication No. WO 2004/023568 A1, LED elements are two-dimensionally connected in series, on an insulation substrate, such as a sapphire substrate, to form LED arrays. Such LED arrays are connected to each other, thereby providing a light emitting device that can be operated at high voltage. Further, such LED arrays are connected in reverse parallel to each other, on the sapphire substrate, thereby providing a single-chip light emitting device that can be operated by an AC power supply.
Since the AC-LED includes light emitting cells formed on the substrate, which is used as a growth substrate, there are limitations to the structure of the light emitting cells and the light extraction efficiency thereof. To solve such problems, a method of fabricating an AC-LED, through a substrate lift-off process, is disclosed in Korean Patent No. 10-0599012, entitled “Light emitting diode employing thermally conductive substrate and method of fabricating the same.”
FIGS. 1 to 4 are cross-sectional views illustrating a conventional method of fabricating a light emitting device. Referring to FIG. 1, semiconductor layers, including a buffer layer 23, an N-type semiconductor layer 25, an active layer 27, and a P-type semiconductor layer 29, are formed on a sacrificial substrate 21. Further, a first metal layer 31 is formed on the semiconductor layers, and a second metal layer 53 is formed on a substrate 51 separate from the sacrificial substrate 21. The first metal layer 31 may include a reflective metal layer. The second metal layer 53 is bonded to the first metal layer 31, so that the substrate 51 is bonded to an upper portion of the semiconductor layers.
Referring to FIG. 2, after bonding the substrate 51, the sacrificial substrate 21 is removed using a laser lift-off process. Further, after the sacrificial substrate 21 is removed, the remaining buffer layer 23 is removed, and the surface of the N-type semiconductor layer 25 is exposed.
Referring to FIG. 3, the semiconductor layers 25, 27, 29 and the metal layers 31, 53 are subjected to a patterning process, using photolithography and etching technologies, to form separate metal patterns 40 and light emitting cells 30 located on the metal patterns 40. Each of the light emitting cells 30 includes a P-type semiconductor layer 29a, an active layer 27a, and an N-type semiconductor layer 25a, which are subjected to patterning.
Referring to FIG. 4, metal wires 57 are formed to electrically connect upper surfaces of the light emitting cells 30 to the metal patterns 40 adjacent thereto. The metal wires 57 connect the light emitting cells 30 to each other, to form series arrays of the light emitting cells. In order to connect the metal wires 57 to the light emitting cells, electrode pads 55 may be formed on the N-type semiconductor layers 25a, and electrode pads may also be formed on the metal patterns 40. Two or more series arrays may be formed and connected in reverse parallel to each other, thereby providing a light emitting diode that can be driven by an AC power source.
As such, the conventional method can improve the heat dissipation of the light emitting device, through appropriate selection of the substrate 51, and can enhance a light extraction efficiency via treatment of the surface of the N-type semiconductor layer 25a. Further, since the first metal layer 31a includes the reflective metal layer to reflect light radiated from the light emitting cells 30 towards the substrate 51, the luminous efficiency may be further improved.
However, such a conventional method may cause a short circuit between the N-type semiconductor layer 25a and the P-type semiconductor layer 29a, due to the adhesion of metallic etching by-products to sidewalls of the light emitting cells 30, during the patterning of the semiconductor layers 25, 27, 29 and the metal layers 31, 53. Further, when etching the semiconductor layers 25, 27, 29, the surface of the first metal layer 31a is exposed and is likely to be damaged by plasma. When the first metal layer 31a includes a Ag or Al reflective metal layer, such etching damage is more pronounced. The plasma damage to the surface of the metal layer 31a deteriorates the contact between the wires 57 or electrode pads, and the metal layer, thereby increasing device failure rates.
In the conventional method, the first metal layer 31 may include the reflective metal layer and thus, may reflect light from the light emitting cells 30 away from the substrate. However, the reflective metal layer is disposed in a space between the light emitting cells 30 and thus, is frequently damaged by etching and/or oxidation, reducing the reflectivity thereof. Furthermore, since the substrate 51 is exposed between the metal patterns 40, light can be absorbed by the substrate 51, thereby causing optical loss. Moreover, since the wires 57 are connected to an upper light emitting surface of the N-type semiconductor layer 25a, light produced by the active layer 25a can be absorbed by the wires 57 and/or the electrode pads 55 located on the light emitting surface, thereby increasing optical loss.